A conventional phase-locked loop is formed by a phase/frequency discriminator connected to a charge pump delivering a voltage to a voltage-controlled oscillator via a filter. The output voltage of the oscillator is delivered to the input of the phase/frequency discriminator via a feedback loop comprising, for example, a divider by N.
The phase/frequency discriminator compares the phase of a reference signal that it receives at its input and the phase of the output signal from the voltage-controlled oscillator, whose frequency is divided by the divider by N of the feedback loop. In another variant, the frequency of the oscillator output signal can also be transposed by using a mixer. The reference signal can, for example, be generated by a quartz oscillator or by another phase-locked loop. Following the comparison, the discriminator generates current pulses, called ‘positive’ and ‘negative’ pulses, whose difference is proportional to the phase difference.
When the positive pulse is at the high logic level, the positive current generated by the charge pump is delivered to the capacitor of the filter connected between the charge pump and the voltage-controlled oscillator such that the output voltage of the filter increases. On the other hand, when the negative pulse is at the high logic level, the negative current from the charge pump is recovered from the filter capacitor such that the filter voltage decreases.
When the two pulses, positive and negative, are simultaneously at the high level, the positive and negative currents cancel, and no current is then either delivered to the filter or flows. When the two pulses, positive and negative, are at the low logic level, the charge pump current sources are then disconnected.
In other words, the charge pump and the filter operate as a digital/analogue converter. The charge pump produces two similar output currents, a positive current and a negative current. The charge pump receives a current when the negative pulse delivered by the phase/frequency discriminator is at its high level and delivers the same current when the positive pulse delivered by the low-frequency discriminator is active. No current flows if the two pulses are at the same logic level, state another way, if the negative and positive currents of the charge pump are exactly equal.
As the frequencies of the two signals, in other words of the reference signal and of the divided signal, tend to equality, the positive and negative pulses delivered by the discriminator become closer and closer and the integration performed by the filter reduces the phase difference to zero. At that moment, no signal is delivered by the discriminator, and the voltage-controlled oscillator is effectively disconnected. This situation is undesirable, since the total gain of the phase-locked loop is reduced to zero, and it then finds itself in a region referred to as a ‘dead band’, within which phase difference correction is no longer effected.
The ideal situation consists of a point-like dead band. However, in practice, this case never occurs given that the switching times of the components are not equal and take place over a given time. In reality, the dead band therefore continues for a certain duration.
In order to pre-empt this critical situation, a first approach involves connecting a leakage resistor of a given value in parallel with the integration capacitor of the filter. However, a resistor with too high a value leads to a higher voltage noise level, which becomes a phase noise at the output of the voltage-controlled oscillator. This approach is therefore not usable in the case of small phase differences between the input signals.
On the other hand, a leakage resistor whose value is not high enough leads to the appearance of spurious frequency lines on either side of the carrier frequency. The reason for this is that by reducing the value of the resistor, the noise is indeed reduced, but the current flowing in this resistor is higher. This current integrated by the capacitor Cf of the filter induces a modulation of the filter voltage that generates a frequency modulation at the output of the voltage-controlled oscillator.
Another approach involves introducing a delay within the phase/frequency discriminator. In this case, the principle involves making the positive and negative pulses delivered by the charge pump go to the high logic level when the respective edges of the reference signal and of the divided signal occur. The positive and negative pulses return to the low level at the end of a predetermined delay.
However, during the time when the pulses are at the logic high level, the charge pump current sources are conducting. Since the sources are never exactly matched, their difference generates a small current that induces a modulation of the filter voltage. As before, this phenomenon is converted into a frequency modulation at the output of the voltage-controlled oscillator.
In addition, each of the aforementioned approaches requires an additional period of injection and/or withdrawal of current from which an additional voltage noise from the filter is generated.